<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0
        }
        
        ul li {
            width: 100px;
            height: 30px;
            line-height: 30px;
            background-color: yellow;
            list-style: none;
            float: left;
            position: relative
        }
        
        div {
            width: 100px;
            height: 100px;
            background-color: red;
            display: none;
            position: absolute;
            left: 0;
            top: 30px;
        }
        
        li:hover {
            background-color: pink
        }
        
        ul li:hover div {
            display: block
        }
    </style>
</head>

<body>

    <ul>
        <li><a href="">菜单栏一</a>
            <div>菜单栏二</div>
        </li>
        <li><a href="">菜单栏一</a>
            <div>菜单栏二</div>
        </li>
        <li><a href="">菜单栏一</a>
            <div>菜单栏二</div>
        </li>
        <li><a href="">菜单栏一</a>
            <div>菜单栏二</div>
        </li>
    </ul>

</body>

</html>